Sonic hedgehog signaling controls the differentiation of motor neurons in the ventral neural tube but the intervening steps are poorly understood. A differential screen of a cDNA library derived from a single Shh-induced motor neuron has identified a novel homeobox gene, MNR2, expressed by motor neuron progenitors and transiently by post-mitotic motor neurons. The ectopic expression of MNR2 in neural cells initiates a program of somatic motor neuron differentiation characterized by the expression of homeodomain proteins, by neurotransmitter phenotype and by axonal trajectory. Our results suggest that the Shh-mediated induction of a single transcription factor, MNR2, is sufficient to direct somatic motor neuron differentiation.
The assembly of neural circuits in the vertebrate central nervous system (CNS) is initiated by the generation of distinct classes of neurons at characteristic positions. The specification of neuronal identity in the CNS appears to be controlled by inductive signals secreted by embryonic organizing centers (Lumsden and Krumlauf, 1996; Tanabe and Jessell, 1996). These signals appear to define neuronal fates by regulating the expression of cell-intrinsic determinants, many of which are transcription factors (Bang and Goulding, 1996). However, the pathways by which specific inductive signals determine the fate of individual neuronal cell types in the CNS are poorly defined. As a consequence, it is unclear whether there are individual transcription factors assigned, in a dedicated manner, to the specification of particular neuronal subtypes or whether the parallel actions of several factors are required.
Spinal motor neurons constitute one subclass of CNS neuron for which some early differentiation steps have been defined (Pfaff and Kintner, 1998). The differentiation of motor neurons depends on spatial signals provided by Sonic Hedgehog (Shh) secreted from the notochord and floor plate (Marti et al. 1995; Roelink et al., 1995; Tanabe et al., 1995; Chiang et al. 1996; Ericson et al. 1996). Shh acts initially to convert medial neural plate cells into a population of ventral progenitors (Ericson et al., 1996) and later directs the differentiation of ventral progenitors into motor neurons and interneurons at distinct concentration thresholds (Roelink et al. 1995; Ericson et al., 1997). The Shh-induced pathway of motor neuron differentiation appears, however, to operate within the context of an independent program of neurogenesis. Neural progenitors that have been exposed to Shh undergo two or more cell divisions before leaving the cell cycle and acquiring motor neuron properties (Ericson et al., 1996). Over this period, ventral progenitors require continued Shh signaling, achieving Shh-independence and committing to a motor neuron fate only late in their final division cycle (Ericson et al., 1996).
Cells in the ventral neural tube respond to graded Shh signaling with the establishment of distinct ventral progenitor populations defined by the expression of the homeodomain proteins Pax6 and Nkx2.2 (Ericson et al., 1997). These two progenitor populations generate distinct classes of motor neurons. Pax6+ progenitors give rise to somatic motor neurons whereas Nkx2.2+ progenitors generate visceral motor neurons (Ericson et al. 1997). As these two progenitor populations leave the cell cycle they express different homeodomain proteins that characterize distinct motor neuron subtypes (Tsuchida et al., 1994; Varela-Echavarria et al., 1996; Ericson et al., 1997; Pattyn et al. 1997). The activity of Pax6 is necessary for the differentiation of somatic motor neurons within the hindbrain (Ericson et al., 1997; Osumi et al., 1997) but it appears that its function is indirect, being required to repress the expression of Nkx2.2 (Ericson et al., 1997).
The dispensibility of Pax6 for somatic motor neuron generation implies the existence of additional genes that determine somatic motor neuron identity. Moreover, the late commitment: of progenitors to a somatic motor neuron fate suggests that the onset of expression of such genes occurs only during the final division cycle of motor neuron progenitors. To identify such determinants a screen for genes expressed by somatic motor neuron progenitors was performed and described, here is the characterization of a novel homeobox gene, MNR2.
MNR2 is expressed selectively by Pax6+ motor neuron progenitors and persists transiently in post-mitotic somatic motor neurons. The ectopic expression of MNR2 in vivo is sufficient to activate a program of somatic motor neuron differentiation characterized by the expression of several homeodomain proteins and Choline Acetyltransferase (ChAT), by the autoactivation of MNR2 and by the extension of axons into ventral roots. This program of motor neuron differentiation is accompanied by the repression of spinal interneuron fates. Thus, the Shh-triggered differentiation of ventral progenitor cells into somatic motor neurons may be directed by the expression of a single homeodomain protein, MNR2.
Introduction
The ability of neurons to form selective neuronal circuits is a function of the molecular properties that they acquire at early stages of their differentiation. The molecular features that distinguish individual classes of neurons appear to control the pattern of axonal projections, the formation of target connections and the expression of specific chemical transmitters. The emergence of a coherent neuronal phenotype is a protracted process and is thought to involve progressive restrictions in the developmental potential of both neural progenitor cells and post-mitotic neurons (Cepko 1999; Edlund and Jessell, 1999). In the peripheral nervous system, convergent programs of transcription factor expression have been suggested to coordinate pan-neuronal properties with more specific aspects of neuronal subtype identity, notably neurotransmitter synthesis and trophic factor sensitivity (Lo et al., 1998, 1999; Pattyn et al., 1999; Goridis and Brunet, 1999).
When and how neuronal subclasses in the central nervous system acquire their specialized functional properties is less well understood. Studies of the differentiation of spinal motor neurons (MNs) have provided some insight into the steps that confer neuronal subtype identity within the central nervous system. Physiological and anatomical studies have revealed that spinal MNs exhibit several levels of organization and function (Landmesser, 1978 a, b) and these have a molecular correlate in the selective patterns of expression of different families of transcription factors (Tanabe and Jessell, 1996; Goulding, 1998). Members of the LIM homeodomain (LIM-HD) protein family define aspects of the generic and columnar identities of spinal MNs (Ericson et al., 1992; 1996; Tsuchida et al., 1994; Sharma et al., 1998). In addition, many of the MN pools that innervate individual muscles in the limb can be defined by the expression of ETS domain proteins (Lin et al., 1998). The analysis of neuronal fate changes that result from the misexpression or inactivation of certain of these nuclear factors has lent support to the idea that they have critical roles in the specification of MN identity (Tanabe et al., 1998: Sharma et al., 1998; see Appel, 1999).
Some of the earlier events that specify the differentiation of neural progenitors into MNs have also been defined. The differentiation of MNs is initiated when progenitor cells located in the ventral half of the neural tube acquire distinct identities in response to the graded signaling activity of Sonic hedgehog (Shh) (Ericson et al., 1996, 1997a,b; Briscoe et al., 1999). The final division of MN progenitors in chick is marked by the onset of expression of two homeodomain proteins, MNR2 and Lim3 (Lhx3) which appear to have distinct roles in MN differentiation (Ericson et al., 1997a; Tanabe et al., 1998; Sharma et al., 1998). MNR2 expression is restricted to MN progenitors whereas Lim3 is expressed by progenitor cells that give rise to an adjacent population of V2 interneurons (Ericson et al., 1997a; Tanabe et al., 1998). In chick, the ectopic expression of MNR2 is sufficient to direct the differentiation of neural cells into MNs and to suppress V2 interneuron generation (Tanabe et al., 1998). In contrast, ectopic expression of Lim3 alone appears to promote the generation of V2 interneurons (Tanabe et al., 1998). These results suggest that MNR2 has a role in specifying whether ventral progenitors that express Lim3 generate MNs rather than V2 neurons.
The function of many of the other transcription factors whose expression is restricted to MNs has not yet been addressed. Amongst these, the homeobox gene Hb9 (Harrison et al., 1994; Ross et al., 1998) is a selective marker of MNs in the developing spinal cord (Pfaff et al., 1996; Saha et al., 1997; Tanabe et al., 1998). Strikingly, HB9 possesses a homeodomain virtually identical to that of MNR2. Moreover, the ectopic expression of HB9 in chick has been shown to mimic the MN-inducing and V2 interneuron repressive activities of MNR2 (Tanabe et al., 1998; unpublished data). In contrast to MNR2, however, the expression of HB9 in chick is excluded from ventral progenitor cells and is restricted to post-mitotic MNs (Tanabe et al., 1998), suggesting that it has a later role in the differentiation of post-mitotic MNs. Further insight into the developmental roles of MNR2 and HB9, however, requires an analysis of MN differentiation in embryos that lack the function of these homeodomain proteins.
To begin to address this issue we have examined MN development in mice in which the Hb9 gene has been inactivated by targeted mutation. In mice lacking Hb9 function, MNs are generated on schedule and in normal numbers. However, soon after MNs have left the cell cycle, there is a dramatic change in the program of MN differentiation. Most strikingly, MNs transiently express transcription factors normally characteristic of V2 interneurons. In addition, and perhaps as a consequence, the transcription factor codes that define the columnar and pool identities of spinal MNs are markedly disrupted. These defects in the transcription factor profile of MNs are accompanied by abnormal MN migratory patterns, by errors in motor axon projections and by defects in the innervation of certain target muscles. Together, these results provide evidence that HB9 has a critical role in the consolidation of MN identity, in particular in repressing the expression of V2 interneuron character.
The initial stages of pancreatic development occur early during mammalian embryogenesis (Wessells et al. 1981) but the genes governing this process remain largely unknown. The homeodomain protein IPF1/PDX1 is expressed in the developing pancreatic anlagen from the ˜10 somite stage (Ohlsson et al. 1993; Ahlgren et al. 1996) and mutations in the IPF1/PDX1 gene prevent the development of the pancreas (Ahlgren et al. 1996; Jonsson et al. 1994; Offield et al. 1996; Harrison et al. 1994). However, the initial stages of pancreatic development still occur in Ipf1/Pdx1 deficient mice (Ahlgren et al. 1993). Hb96 is a homeobox gene that in humans has been linked to dominant inherited sacral agenesis (Ross et al. 1998) and we show here that HB9 is expressed at early stages of mouse pancreatic development and later in differentiated -cells. Hb9 has an essential function in the initial stages of pancreatic development. In absence of Hb9 expression, the dorsal region of the gut epithelium fails to initiate a pancreatic differentiation program. In contrast, the ventral pancreatic endoderm develops but exhibits a later and more subtle perturbation in -cell differentiation and in islet cell organisation. Thus, dorsally Hb9 is required for specifying the gut epithelium to a pancreatic fate and ventrally for ensuring proper -cell differentiation.